This invention relates to a process of purifying a liquid, particularly water, using a thermoelectric module; and apparatus of use in said process.
Thermoelectric modules are small, solid state, heat pumps that cool, heat and generate power. In function, they are similar to conventional refrigerators in that they move heat from one area to another and, thus, create a temperature differential.
A thermoelectric module is comprised of an array of semiconductor couples (P and N pellets) connected electrically in series and thermally in parallel, sandwiched between metallized ceramic substrates. In essence, if a thermoelectric module is connected to a DC power source, heat is absorbed at one end of the device to cool that end, while heat is rejected at the other end, where the temperature rises. This is known as the Peltier Effect. By reversing the current flow, the direction of the heat flow is reversed.
It is known that a thermoelectric element (TEE) or module may function as a heat pump that performs the same cooling function as Freon-based vapor compression or absorption refrigerators. The main difference between a TEE device and the conventional vapor-cycle device is that thermoelectric elements are totally solid state, while vapor-cycle devices include moving mechanical parts and require a working fluid. Also, unlike conventional vapor compressor systems, thermoelectric modules are, most generally, miniature devices. A module typical measures 2.5 cmxc3x972.5 cmxc3x974 mm, while the smallest sub-miniature modules may measure 3 mmxc3x973 mmxc3x972 mm. These small units are capable of reducing the temperature to well-below water-freezing temperatures.
Thermoelectric devices are very effective when system design criteria requires specific factors, such as high reliability, small size or capacity, low cost, low weight, intrinsic safety for hazardous electrical environments, and precise temperature control. Further, these devices are capable of refrigerating a solid or fluid object.
A bismuth telluride thermoelectric element consists of a quaternary alloy of bismuth, tellurium, selenium and antimonyxe2x80x94doped and processed to yield oriented polycrystalline semiconductors with anisotropic thermoelectric properties. The bismuth telluride is primarily used as a semiconductor material, heavily doped to create either an excess (n-type) or a deficiency (p-type) of electrons. A plurality of these couples are connected in series electrically and in parallel thermally, and integrated into modules. The modules are packaged between metallized ceramic plates to afford optimum electrical insulation and thermal conduction with high mechanical compression strength. Typical modules contain from 3 to 127 thermocouples. Modules can also be mounted in parallel to increase the heat transfer effect or stacked in multistage cascades to achieve high differential temperatures.
These TEE devices became of practical importance only recently with the new developments of semiconductor thermocouple materials. The practical application of such modules required the development of semiconductors that are good conductors of electricity, but poor conductors of heat to provide the perfect balance for TEE performance. During operation, when an applied DC current flows through the couple, this causes heat to be transferred from one side of the TEE to the other; and, thus, creating a cold heat sink side and hot heat source side. If the current is reversed, the heat is moved in the opposite direction. A single-stage TEE can achieve temperature differences of up to 70xc2x0 C., or can transfer heat at a rate of 125 W. To achieve greater temperature differences, i.e up to 131xc2x0 C., a multistage, cascaded TEE may be utilized.
A typical application exposes the cold side of the TEE to the object or substance to be cooled and the hot side to a heat sink, which dissipates the heat to the environment. A heat exchanger with forced air or liquid may be required.
Water in bulk may be purified by a number of commercial methods, for example by reverse osmosis and by distillation processes.
Reverse osmosis (R.O.) technology relies on a membrane filtration system that is operated under high pressure. While this technology is one of the two leading technologies of water purification, it suffers from the following main disadvantages:
(a) the infrastructure of the system is complex because of the operating pressure, typically 8 atmospheres, required to cause the reverse osmosis process in the membrane;
(b) the membrane is an expensive component that needs to be replaced, frequently, depending on the salinity and the purity of the source water, generally, every 4 to 6 months. Also, there is a problem of membrane fouling, if the quality of the source water is not within certain bounds. The restriction on the water quality that is inputted into the system precludes many sources of water or would necessitate the utilization of pretreatment systems;
(c) the amount of purified water is very low when compared to the amount of water that has to be pumped into the system. Therefore, the cost of pumping and discharging the rejected water (capital cost to install the required facility and the energy cost to operate and maintain it) makes this system very costly;
(d) the quality of purified water obtained by the reverse osmosis process is inferior to that of distilled water, in the sense that it leaves small microorganisms and any impurities that are small enough to go through the membrane. Also, as the membrane ages, the water quality does not remain consistent;
(e) the system is feasible from a physical and economical point of view, for only large commercial installations. The system is not amenable for use in household units or even in small commercial units; and
(f) energy, operating and maintenance costs are high for the R.O. system.
The main disadvantages of distillation technologies, such as the multistage flashback evaporation systems, are:
(i) relatively large capital cost needed to assemble and install the system;
(ii) high energy costs to perform the evaporation, provide energy and equipment for the vacuum system and the condensation in, literally, three independent subsystems;
(iii) significant corrosion problems that necessitate significant pretreatment of input water and complete replacement of plant equipment as frequently as every three to four years;
(iv) the system, generally, needs to be installed only near large power plants and large bodies of water; and
(v) the disadvantages listed in item (e) and (f) hereinabove.
There is, therefore, a need to provide a means for producing a purified liquid, particularly water, in a safe, reliable, convenient, relatively cheap manner, having low energy requirements, and which either eliminates or reduces the aforesaid disadvantages.
Offenlegungsschrift DE 35 39 08 6A (Wagner Finish Tech Center GmbH) published May 7, 1987, describes apparatus for the purification of organic solvents containing paint or varnish by evaporation and condensation by use of a Peltier element which functions as both a heating and cooling element during the evaporation and condensation stages. An essential feature is the condensation of the solvent vapor solely on the cooling element.
It is known that in addition to the production of a temperature differential across the module between the xe2x80x98hotxe2x80x99 and xe2x80x98coldxe2x80x99 surfaces that heat may be beneficially xe2x80x9cpumpedxe2x80x9d from the cold surface to the hot surface through the module. For example, latent heat of condensation of a vapor on the cold surface may be captured by the cooler element and pumped to the hot side. It is also known that the heat pumped by the cold side varies linearly with the cold side temperature.
However, in the apparatus and process described in OLS DE3539086A a balanced continuous evaporation and condensation equilibrium cannot be established by reason that the cold side of the module absorbs the latent heat which is then pumped to the hot element and, thus, very significant amounts of latent heat of the steam generated upon condensation must be removed from the vessel or the process xe2x80x98shut downxe2x80x99, intermittently, for periods of time to prevent the hot element overheating. This is an unsatisfactory situation when continuous process conditions are desired.
Japanese Kokai JP 07 209841, published Aug. 11, 1995 to Koicki Hayashi describes a small, low-cost and high-efficiency developer waste solution concentrator for use in small-scale retail stores. The concentrator is provided with a concentrating tank divided into an evaporation tank and a condensation tank, the upper parts of these tanks being in communication with-each other; a heat-generating/heat-absorbing section which is made up of Peltier element parts wherein a heat-generating side is in contact with the evaporation tank and a heat-absorbing side with the condensation tank; a replenishing pump to control the volume of waste solution in the evaporation tank to a liquid volume within the certain constant range; and a cooling section to control the temperature of the heat-generating element side to a value within a certain constant range. The embodiments described therein effect condensation on the cooler element side of the module surface to provide purified liquid. To maintain the hot surface of the element at the desired temperature, a cooling fan means in conjunction with heat release fins are provided. However, such heat control means results in the need for additional physical items and reduced electrical and thermal energy efficiency.
There is, therefore, a need for a simple, safe, convenient and reliable process operable under continuous conditions of purifying a liquid, particularly water.
It is an object of the present invention to provide a method and apparatus for producing a purified liquid, particularly water under continuous conditions in a safe, convenient, reliable and relatively cheap manner by means of thermoelectric modules to generate hot and cold elements.
Accordingly, in one aspect the invention provides an improved, continuous process for treating an impure liquid to produce purified liquid, said process comprising electrically activating a thermoelectric module to provide a first heated surface and a cooler surface; feeding said impure liquid to said first heated surface to produce vapor of said liquid; and transferring said vapor to said cooler surface to effect heat transfer to said cooler surface, the improvement comprising (a) directing a minor portion of said vapor adjacent to or onto said cooler surface to maintain said cooler surface at a temperature at or near to the boiling point of said liquid; (b) directing a major portion of said vapor to condensation means comprising a second cooler surface to effect heat transfer to said second cooler surface and condensation of said vapor to produce said purified liquid and collecting said purified liquid.
In this specification and claims the term xe2x80x9cheatable or heated surfacexe2x80x9d means a surface of said thermoelectric module which is heated when said module receives an electric current or a surface in thermal communication with said module as to be heated thereby. The term xe2x80x9ccoolable or cooled surfacexe2x80x9d means a surface of said module which is cooled when said module receives an electric current or a surface in thermal communication with said module as to be cooled thereby.
By the term xe2x80x9ccontinuous processxe2x80x9d in this specification is meant a process as defined that does not need, of necessity, to be intermittently stopped or slowed, or requiring auxiliary cooling of the thermoelectric module in order to prevent overheating of the module.
The term xe2x80x9cminor portionxe2x80x9d means less than half of the vapour or steam generated by the hot element of the module, and which is a function of the design of the apparatus and operating conditions as to prevent overheating of the module by excessive heat transfer to the cooler element.
Generally less than about 40% of the steam is directed to the cooler element.
In a particularly valuable aspect, the liquid is water, and by the term xe2x80x9cimpure waterxe2x80x9d is herein meant water containing impurities such as, for example, dissolved salts and other matter, and/or suspended particulate matter which impure water may be evaporated and concentrated without unwanted carry-over of such impurities.
The term xe2x80x9cvaporxe2x80x9d includes xe2x80x9csteamxe2x80x9d.
Preferably, the invention provides process for treating an impure liquid to produce purified liquid, said process comprising
(i) electrically activating a thermoelectric module to provide a heated surface in a first chamber and a cooler surface in a second chamber;
(ii) feeding said impure liquid to said heated surface to produce liquid vapour;
(iii) directing said liquid vapor from said first chamber to said second chamber;
(iv) contacting a minor portion of said liquid vapor with said cooler surface to effect heat transfer to said cooler surface;
(v) cooling a major portion of said vapor in said second chamber with condensation means comprising a second cooler surface to effect heat transfer from said vapor to said second cooler surface and condensation of said vapor to provide said purified liquid; and
(vi) collecting said purified liquid.
In a further aspect, the invention provides an improved liquid purifier for purifying a liquid under continuous operating conditions comprising thermoelectric module means having a first heatable surface and a coolable surface; means for contacting said impure liquid with said heatable surface to produce vapor of said liquid; means for transferring said vapor to effect heat transfer to said coolable surface; means for condensing said vapor to said purified liquid; and means for collecting said purified liquid; the improvement comprising (a) means for directing a minor portion of said vapor to said coolable surface to maintain said cooler surface at a temperature near or at the boiling point of said liquid; (b) condenser means comprising a second coolable surface means to effect heat transfer to said second coolable surface and, consequently, condensation of a major portion of said vapor to produce said purified liquid; (c) means for directing said major portion of said vapor to said condenser means; and (d) means for collecting said purified liquid from said condenser means, wherein said thermoelectric module is adapted to receive an electric current to activate said module to heat said heatable surface and cool said coolable surface.
In prior art apparatus, the coolable surface constitutes the means for condensing the vapor to purified liquid.
The means for directing vapor to the coolable surface may, include, for example, merely, conduit, passage, guide or the like which allows a portion minor of the vapor to pass to the cold sink.
In one embodiment, the invention provides a liquid purifier comprising a housing having a first chamber and a second chamber; divider means separating said first and second chamber one from the other; said divider means comprising a thermoelectric module having a heatable surface received within the first chamber and a coolable surface within the second chamber; means for contacting impure liquid with said heatable surface within said first chamber to produce vapor; first transfer means for directing a minor said portion of vapor to said coolable surface to effect heat transfer to said coolable surface to maintain the temperature of said coolable surface at or near the boiling point of said liquid; condenser means having a second coolable surface for condensing a major portion of said vapor within said second chamber by heat transfer to produce said purifier liquid; second transfer means for directing said major portion of said vapor by means to said condenser means; and means for pre-heating said impure liquid feed by said heat transfer with said condenser means; and wherein said thermoelectric module is adapted to receive an electric current to activate said module to heat said heatable surface and cool said coolable surface.
Most preferably, the apparatus has a plurality of the thermoelectric modules aligned coplanar within a divider between the chambers and/or within one or more walls of the chamber.
Thus, preferably, a plurality of modules are arrayed in coplanar fashion in a planar member to provide, for example, a plurality of heatable surfaces at one, i.e. top face of the module and a plurality of coolable surfaces on its bottom face. The aforesaid top face may constitute an inner face of an evaporation chamber and the aforesaid bottom face constitute the corresponding outer face of the evaporation chamber.
Thus, the essence of the present invention is to achieve a continuous evaporation and condensation equilibrium within the apparatus by removing the majority of the thermal (latent) energy of the vapor remote from the coolable surface of the module. The presence of this remote secondary condenser surface which, preferably, effects heat transfer of at least about 60% from the steam enables satisfactory continuous removal of excess heating power from the module. In one embodiment, a 100% conversion from vapor to liquid within the same chamber containing the module cold side can be achieved.
Maximum efficiency of the heat pump can be achieved by maintaining the temperature of the module coolable surface essentially at the boiling point of the liquid, i.e. 97xc2x0-100xc2x0 C. for water by means of a suitable minor portion acting on the coolable surface to effect suitable, but not excessive heat transfer.
In a most preferred aspect, the heat transferred from the major portion of condensed vapor with the condenser is used to pre-heat the impure liquid feed, preferably from ambient temperature to a temperature of at least 90xc2x0 C. in the case of water. This preheating of the feed water increases the electrical utilization efficiency to over 167% and provide significant power savings when compared to, say, 95% power utilization efficiencies achieved with prior art conventional water purifiers, hereinbefore described.
Thus, the present invention provides in one aspect a water purification system which provides the advantages of:
(a) providing both water evaporation and cooling within the same unit;
(b) being significantly energy efficient by the use amount of electrical energy and heat transfer to perform evaporation and condensation; which energy utilization does not exist in any of the water purification technologies known at this time;
(c) recovering all of the water inputted into the system as pure water, without having to discharge water with high concentrations of impurities and salt as is the case in reverse osmosis technology;
(d) portability of the system and its ability to be scaled up over a very wide range of dimensions and capacities; and wherein the capacity of the system can be increased in a modular fashion;
(e) having the ability to energize the system from a very wide variety of power sources, such as, for example, operable throughout in the world, including remote areas that are not even connected to an energy generation grid; and
(f) having the ability of the system to handle any type of water regardless of its salinity and impurities, while still producing pure water that has the same quality as distilled water, which is free from all organic, non-organic and microbial elements.
I have found that non-insulated surfaces of vapor-receiving chambers, conduits and the like enhance condensation of the vapor to reduce the load on the module colder surface. This advantageous arrangement can be enhanced by passing the feed liquid through or around the xe2x80x9ccoldxe2x80x9d chamber to enhance condensation external of the module cooler surface and also pre-heat the feed water.
It is a further aspect of the present invention to provide a plurality of multimodule units in the form of an assembly, which may be so designed to be of modular construction as to be built-up to any desired operating size.
The apparatus according to the invention may be operated over significant periods of time although there may be a build-up of impurities in the evaporation tray of the hot side surface of the module which may require down-time cleaning.